Treat empty pipelines as pass-through

[sophiajt]

Description

This allows empty pipelines to pass their emptiness through a filter. This helps fix issues like trying to run a filter on an ls in an empty directory. It also feels a bit more reasonable that a filter filters what is there but doesn't require something to be there.

fixes #8393

User-Facing Changes

No breaking changes (that I know of). Should allow filtering to be a little less surprising with emptiness.

Tests + Formatting

Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace to check that all tests pass

After Submitting

If your PR had any user-facing changes, update the documentation after the PR is merged, if necessary. This will help us keep the docs up to date.

[codecov]

Codecov Report

Merging #8395 (b77bc45) into main (ca275f5) will decrease coverage by 0.08%.
The diff coverage is 50.00%.

❗ Current head b77bc45 differs from pull request most recent head 65f166c. Consider uploading reports for the commit 65f166c to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8395      +/-   ##
==========================================
- Coverage   68.54%   68.46%   -0.08%     
==========================================
  Files         636      620      -16     
  Lines      102281    99520    -2761     
==========================================
- Hits        70110    68138    -1972     
+ Misses      32171    31382     -789     

Impacted Files	Coverage Δ
crates/nu-protocol/src/pipeline_data.rs	77.99% <50.00%> (+2.48%)	⬆️

... and 264 files with indirect coverage changes

[sophiajt]

I want to also land some corresponding fixes to some of the filter commands, so we have a consistent way filtering works with empty pipelines

[fdncred]

@jntrnr Thoughts on this one? We have time before the next release if we land soon.

[fdncred]

pinging you again @jntrnr so that, if we want this, we can land it shortly after the next release in a few days.

[amtoine]

💡 #8393 has been fixed around the same time independently in #8439 👌

i think the two approaches are complementary and should be kept ✨

[sholderbach]

Marking as a breaking changes as this could be a potential change to the semantics of erroneous usage of filtering commands after "void" commands that return PipelineData::Empty

[amtoine]

@sholderbach
you're absolutely right, this is a breaking change, mainly on error handling

> cd . | where blah
Error: nu::shell::only_supports_this_input_type

  × Input type not supported.
   ╭─[entry #13:1:1]
 1 │ cd . | where blah
   ·        ──┬──┬
   ·          │  ╰── input type: null
   ·          ╰── only list, binary, raw data or range input data is supported
   ╰────

now becomes silent... 😕

> cd . | where blah
╭────────────╮
│ empty list │
╰────────────╯

[amtoine]

if we want things like cd . | where blah to return an error (i think it should), we should probably revert that and add a test for this 🤔